(1) Field of the Invention
The present invention relates to a polyester having an excellent photopolymerization-initiating property to a radical-polymerizable monomer. More particularly, the present invention relates to a polyester having a dithiocarbamate group as the photofunctional group.
(2) Description of the Related Art
A high polymer generally called "polyester", which is obtained by polycondensation of a diol with a dicarboxylic acid, a dicarboxylic acid chloride or a dicarboxylic acid anhydride, is practically valuable because the physical properties can be adjusted within a wide range according to the kinds of monomers used, the molecular weight can be easily controlled and it has excellent transparency, surface hardness, heat resistance, electric resistance characteristics, chemical resistance, weatherability and durability. From the practical viewpoint, the polyester is roughly divided into a saturated polyester and an unsaturated polyester. The saturated polyester is represented by polyethylene terephthalate (PET) and is widely used as synthetic fibers, engineering plastics, elastomers and toners. The unsaturated polyester is represented by a resin synthesized from ethylene glycol and maleic acid and is a high polymer having a double bond in the main molecular chain, and the unsaturated polyester is widely used in the state dissolved in an ethylenically unsaturated monomer as a casting resin, a paint resin, a resin for a decorative board, a resin for a button or a resin for glass fiber-reinforced plastics (see Plastic Techniques, volume 13, written by Tsutomu Akita, published by Kogyo Chyosakai, 1970 ).
It is expected that the polyester will be applied to use in the fields where an optical reaction such as photo-curing, photo-forming or photopolymerization is utilized, for example, a photoresist, a photo-curable ink or a photo-curable paint.
A polymeric initiator can be used as an intermediate material for a block copolymer and a graft copolymer. Accordingly, a polyester photopolymerization initiator can also be expected as an intermediate material for a polyester type block copolymer or graft copolymer.
As the conventional means for applying a polyester to the photopolymerization system, there is known a process in which a radical-polymerizable monomer and a photosensitizer are added to a polyester having a double bond mainly in the main molecular chain and the composition is irradiated with light to impart a crosslinked structure and obtain a molded article, a coating and the like (see "Photopolymer" compiled by Photopolymer Symposium and Editorial Committee, CMC, 1977, page 228 et seq).
According to the conventional technique, however, since the double bond cannot be specifically located at terminals of the main molecular chain, it is difficult to synthesize a block copolymer. Furthermore, if a crosslinked product of a polyester is synthesized, the radical-polymerizable monomer to be copolymerized is homopolymerized and a polymer not participating in the crosslinked structure is left separately from the polyester. It is considered that a mixture of a crosslinked polyester and a linear polymer is finally formed. It also is considered that the incorporated linear polymer has bad influences on the mechanical properties such as strength at break and impact strength and the chemical properties such as chemical resistance and water resistance in the formed crosslinked body.